1. Field of the Invention
The present invention relates to a spindle motor driving circuit using PWM (pulse width modulation) control and VM (applied voltage) control.
2. Description of the Related Art
FIG. 5 is a block diagram showing a conventional spindle motor driving circuit which achieves general VM variable linear drive.
A spindle motor driver 51 does not have a circuit for controlling a duty cycle, but has an electric current setting amplifier 511 and a VM selecting circuit 512, which differ from the PWM control method which is described below.
The linear drive method directly controls an output electric current of an output three-phase bridge depending on an input voltage, to drive the spindle motor at a predetermined speed. In the linear drive method, an electric current setting amplifier 511 outputs a signal depending on a signal VREF2-VIN, and this output controls a phase excitation circuit 11.
A circuit (using two power sources) in FIG. 5 has two systems of VM power sources in a switching regulator 52. In response to an external logic signal, a VM selecting circuit selects one of VM1 and VM2 as the VM power source.
FIG. 6 shows the structure of the conventional PWM control spindle motor driving circuit, and FIG. 7 shows motor speed/input differential voltage characteristics.
The PWM control spindle motor driving circuit shown in FIG. 6 comprises a spindle motor driver 61, a switching regulator 62, and a control circuit 63.
The spindle motor driver 61 comprises a phase excitation circuit 11, a duty cycle setting comparator 616, a reference power source (VREF3) 19, a triangular wave oscillation circuit 17, and a three-phase bridge circuit 12 controlled by an excess current restriction comparator 18 and a phase excitation circuit 11.
A differential voltage between VREF2 and VIN of the control circuit 63 is input to the duty cycle setting comparator 616 in the spindle motor driver 61, and is compared with triangular waves from the triangular wave oscillation circuit 17, to thereby produce rectangular waves. The rectangular waves are supplied as the gate voltage of the three-phase bridge circuit 12. The PWM switching of the three-phase bridge circuit 12 is performed based on the gate voltage, and the electric current in the spindle motor M is thus adjusted so that the characteristics shown in FIG. 7 are achieved.
The linear driving method shown in FIG. 5, however, increases the electric power consumption because the method drives the motor based on the electric current and because this electric current flows continuously. Therefore, heat production is increased when driving the motor at a high speed or when starting the motor.
The PWM driving method shown in FIG. 6 eliminates the problem of the power consumption, but makes the setting of the duty cycle (the on-duty cycle setting) around the minimum speed of the motor and around the maximum speed difficult. Therefore, a dead zone occurs depending on the off-set voltage of the circuit, the linearity characteristics deteriorate, the control of speed around the minimum and maximum rotation speeds becomes difficult, and therefore the access time is lengthened when the motor is used in a disk drive.
It is therefore an object of the present invention to provide a spindle motor driving circuit which can operate linearly from the minimum rotation speed to the maximum rotation speed.
In the first aspect of the present invention, the spindle motor drive circuit comprises: a power source voltage varying device for varying the voltage applied to the motor by controlling the power source voltage; and a pulse width varying device for varying the pulse width by pulse width modulation.
In the second aspect of the present invention, the power source voltage varying device varies the voltage around a minimum rotation speed of the motor or around a maximum rotation speed of the motor.
In the third aspect of the present invention, the power source voltage device sets the minimum pulse width or the maximum pulse width as the fixed pulse width.
In the fourth aspect of the present invention, the applied voltage and the pulse width are controlled based on the differential voltage between the control reference voltage and the control input voltage.
According to the present invention, in the high duty cycle and the low duty cycle regions, the applied voltage is controlled. In the other regions, the applied voltage is fixed, and PWM control is performed. Therefore, the input dead zone can be eliminated, and the linearity of the characteristics from low rotation speeds to high rotation speeds can be improved.
Further, in the high duty cycle region, the applied voltage is increased so that the maximum rotation speed is increased. Therefore, the maximum torque can be advantageously increased.